Sun-Earth Day 2009: Our Sun Yours to Discover

Issue #69: Star Mass, from Brown Dwarfs to Supermassive Stars

Figure 1: The Pistol Star near the galactic core as seen by the Hubble NICMOS camera in 1997 (Courtesy: Dr. Don Figer(UCLA) and NASA/HST)

Although space is infinite, stars do not come in an infinity of sizes!

The basic definition for a star is that it is a collection of matter that generates its own light through thermonuclear fusion. A byproduct of this is that the heat energy produces enough internal pressure in the star to prevent further gravitational collapse. Detailed physical models for stars that specify all of the kinds of fusion reactions that can occur among the 100+ elements in the periodic table, allow astrophysicists to build stars in their computers. The fusion reaction that requires the lowest temperature to 'jump start' the fusion involves the element deuterium and requires a temperature of just under 1 million K.

The lowest-mass brown dwarf has a mass of just above 13 times the mass of Jupiter. The most massive brown dwarf is about 70 times the mass of Jupiter. They range in temperature from 700K to 2,500 K for the more massive dwarfs. Unlike normal stars, however, it is not the 'thermal pressure' from nuclear reactions that holds-up a brown dwarf. In their interiors, electrons get so crowded together that 'electron degeneracy pressure' becomes more important: the same pressure that holds-up white dwarf stars.

More massive bodies can achieve higher core temperatures, which makes deuterium fusion run faster, but also allows for the fusion of lithium at about 65 times the mass of Jupiter. Hydrogen fusion, which defines the main energy source for normal stars like our sun, commences at about 90 Jupiter masses (0.09 solar masses).

Brown dwarfs were first proposed in 1975, but it took several decades of searching in order for astronomers to finally locate the first verifiable candidate. It was found in 1988 by astronomers looking for white dwarfs. This particular body orbited the white dwarf star known only by its catalog number 'GD-165'.

Since 1975, astronomers have discovered hundreds more of these once-illusive dwarf stars, and now group them into two families because of their very different temperatures and spectra.

L-dwarfs - (Example: GD-165B) These stars are cool enough that their spectra show very strong absorption by water and carbon monoxide. 400 of these have been discovered, mostly by the 2MASS infrared survey because the stars are bright infrared objects, though faint optically.

T-dwarfs (Example: Gleise 229B and Figure 2) These stars are even cooler that L-dwarfs and have very strong spectral absorption caused by methane. 58 of these objects are now known.

On the other end of the mass scale, we have the hypergiant stars with masses between 100-200 times the sun. A galaxy like the Milky Way, with its 200 billion stars may have only a few dozen of these heavy-weights! One such star, the Pistol Star (Figure 1), is near the core of the Milky Way and is 10 million times as luminous as our sun, and 100-tims as massive.

The Eta Carina nebula is the home to a dying, massive star about 100 times the sun's mass. The titanic explosions that have rocked this star in the last 3200 years signal that it is near the end of its life, and may erupt as a 'hypernova' in the near future; perhaps within the next million years.

Enormous stars like Eta Carina take only a few million years to be born, and then explode as supernovae, but with a big difference from ordinary supernovae. Their cores are so massive that they implode into black holes orbited by dense disks of matter, which beam out intense bursts of gamma-rays that can be seen as flashes clear across the universe. Any planet caught in these gamma-ray beams will probably have its atmosphere completely ionized, and any life there will end instantly.

Although such massive stars are rare in today's universe, they were once the most common type of star just after the end of the cosmic Dark Ages about 10 million years or so after the Big Bang. These Population III stars exploded within a few million years, and littered the universe with elements like oxygen, iron, nitrogen and other 'heavy elements'. Without these ancient stars, planets and organic life would never have occurred due to the lack of the necessary elements. NASA's telescopes and satellites, such as Hubble, Spitzer, Chandra and eventually the Webb Space Telescope, continue the search for these ancient stars to test our most advanced theories about the evolution of our universe and its contents.

The Sun-Earth Connection

The realization that the sun is a star places it within the range of masses of other stars in the sky, mid-way between the least massive and most massive known stars.

Space Math Connection:

Super-Fast Solar Flares [Problem 101]

Students will analyze consecutive images taken of an erupting solar flare, and use the information to calculate the speed of the flare. [Grade 6-9; Skills: image scales; time calculation; speed calculation]

Finding Mass in the Cosmos [Problem 212]

Students work with a simple equation to determine the mass of the Earth, Sun and the Milky Way. [Skills: Algebra II; working with equations and evaluating them for specific values of the variables]

The Mass of the Moon [Problem 178]

Students use data from the Apollo-11 mission to determine, from the period and altitude of the Command Module, the mass of the moon. [Skills: Algebra II; working with equations and evaluating them for specific values of the variables]